1. Field of the Invention
The present invention relates to a variable magnification optical system, and particularly to a variable magnification optical system suitable for use as a projection optical system in a projection apparatus (for example, a liquid crystal projector for projecting an image from a display device such as a liquid crystal panel onto a screen).
2. Description of the Prior Art
In recent years, as the use of personal computers prevail, liquid crystal projectors are in wider and wider use as a presentation tool in business and other scenes. On the other hand, an increasing demand is expected for liquid crystal projectors that are used at home to realize so-called home theaters. Conventionally, for such uses, single-panel or three-panel liquid crystal projectors of a front-projection type, having resolutions of 100,000 to 300,000 pixels, have been introduced on the market.
Variable magnification optical systems for use as projection optical systems in such liquid crystal projectors are proposed, for example, in Japanese Laid-open Patent Applications Nos. H7-218837 and H8-201690. Both of these Patent Applications disclose variable magnification optical systems consisting of three lens units, and more particularly positive, negative, and positive lens units, and these optical systems offer satisfactory optical performance as long as they are used to project images obtained from conventional display devices. However, these optical systems are not suitable for use with display devices having higher resolutions, because the latter demand more thorough reduction of lateral chromatic aberration than can typically be achieved in the former.
A variable magnification optical system that suffers from relatively small lateral chromatic aberration is proposed in Japanese Published Patent No. H3-58490. This optical system, designed for enlargement of film images, consists of three, i.e. positive, negative, and positive lens units, and is so constituted that variation of lateral chromatic aberration as occurs while the magnification is being varied for adjustment is reduced by a negative lens element made of anomalous-dispersion glass provided in the second lens unit. However, even this optical system does not achieve sufficient reduction of lateral chromatic aberration as is required to properly project images obtained from higher-resolution display devices.
Nowadays, higher image quality is sought after in liquid crystal projectors. For example, for presentation purposes, projectors having resolutions higher than 800.times.600 pixels (SVGA) are preferred to those having a conventional 640.times.480 pixels (VGA). Even in home-use projectors, horizontal resolutions higher than 400 scan lines are preferable, now that high-definition television services such as Japan's Hi-Vision have gone into operation. To satisfy such requirements for higher image quality, display devices such as liquid crystal panels have come to offer higher resolutions and, in line with this, liquid crystal projectors have come to typically adopt a three-panel design employing a dichroic prism. In addition, to make projectors as compact as possible, display devices such as liquid crystal panels are made smaller and smaller. To achieve proper projection of images obtained from such a display device that has a smaller size and offers a higher resolution than ever, it is essential to use a variable magnification projection optical system that offers higher optical performance than ever. However, as noted previously, conventional variable magnification projection optical systems do not offer sufficiently high optical performance (in particular, lateral chromatic aberration correction performance) for such applications.